Wireless Local Area Networks (WLANS) have become ubiquitous. In wireless Local Area Networks (LANS), a wireless channel can be reserved for the transmission of a sequence of frames while employing asynchronous distributed random channel access methods. In such an environment, both the source and destination of the transmission broadcast the reservation duration in order to establish the interference neighborhood.
According to the 802.11 distributed channel access MAC protocol, RTS/CTS frames are used to notify neighbors of a transmission. The source and destination of the transmission the RTS and CTS frames, respectively, which contain the reservation duration in order to establish the interference neighborhood. Nodes receiving either frame refrain from transmitting or agreeing to receive a transmission for the specified duration. This way, ‘hidden terminals’, that is nodes that cannot hear the transmission by may cause interference to the receiving node, will refrain from transmitting for the duration indicated in the received CTS frame.
One method of reserving the channel and preventing collisions with hidden terminals in wireless LANS is by utilizing Frame-by-frame reservation. RTS/CTS frames are sent at the start of the reservation. The reservation time is extended on a frame-by-frame basis, by updating the duration of the reservation with each data frame and the acknowledgement that follows. A consequence of frame-by-frame reservation if that, if the reservation is not accepted—that is, a CTS frame is not returned in tine—there is no reserved time left unused and, hence, the reservation does not require cancellation.
Another method of performing reservation in wireless LANS is by utilizing Start-to-finish reservation. A start-to-finish reservation applies to any combination of nodes (i.e. mesh points/APs/stations). A node reserves a channel to cover an entire sequence of transmissions, directed to a single destination, possibly including responses from the destination. Under this method, the channel may be reserved by setting the duration field value to the time it will to transmit the entire sequence of frames. If the reservation request is not authorized—that is, a CTS frame is not returned in tine—or if time remains reserved at the completion of transmission, the reservation must be cancelled.
Cancellation of a reservation can be problematic in an 802.11 WLAN, because a node relies on a single timer, the NAV, in order to determine if the channel is available for transmission. When a node receives a frame with a value in the Duration field that would extend the occupancy of the channel beyond that of prior reservations, the NAV timer is updated to reflect the longer reservation. It is possible for a node to receive multiple NAV-setting frames from different sources. For instance, a node may receive a CTS frame from one neighbor, and an RTS from another neighbor, who has not received the CTS. Cancellation of the transmission associated with the RTS could be achieved either by listening for an idle channel, or by receiving an explicit cancellation of the reservation from the source of the RTS. If a node resets its NAV based on such a cancellation, it risks causing interference/collision to the neighbor that had sent the CTS frame if this neighbor is still receiving the transmission associated with that CTS frame. Because a single NAV timer is maintained, independently of the source of the NAV-setting request, there is no way to determine whether cancellation of a reservation should cause the NAV timer to be reset.
In spite of this deficiency, the start-to-finish NAV setting method is important because it affords protection from ‘hidden terminals’, a problem that is exacerbated in mesh networks because of the long distances between neighbors.
WLANS can include multiple channels and multiple radios. An approach that offers both multi-channel and multi-radio capabilities appears in the co-pending patent application titled “A Protocol For Wireless Multi-Channel Access Control”, Ser. No. 11/393,127, filed Mar. 29, 2006, the disclosure of which is incorporated by reference herein, which describes the CCC (Common Control Channel) MAC protocol. CCC utilizes two types of logical channels, the control channel and the data channels.